Abstract
A new three-dimensionally preserved troodontid specimen consisting of most of the skull, partial mandibles and six articulated cervical vertebrae (PMOL-AD00102) from the Early Cretaceous Yixian Formation of Beipiao, western Liaoning, China is identified as Sinovenator changii on the basis of a surangular with a “T”-shaped cross-section. High-resolution computed tomographic data for the skull of this new specimen facilitated a detailed description of the cranial anatomy of S. changii. New diagnostic features of S. changii include a well-developed medial shelf on the jugal, a slender bar in the parasphenoid recess, a lateral groove on the pterygoid flange of the ectopterygoid, and the lateral surface of the anterior cervical vertebrae bearing two pneumatic foramina. Our new observation confirms that the braincase of Sinovenator is not as primitive as previously suggested, although it still shows an intermediate state between derived troodontids and non-troodontid paravians in having an initial stage of the subotic recess and the otosphenoidal crest. Additionally, this new specimen reveals some novel and valuable anatomical information of troodontids regarding the quadrate-quadratojugal articulation, the stapes, the epipterygoid and the atlantal ribs.
Highlights
Troodontidae is a group of small to middle-bodied theropod dinosaurs, and is well known from the Cretaceous rocks of Asia and North America (Makovicky & Norell, 2004)
Many exquisitely preserved troodontid fossils have been reported from the Early Cretaceous Jehol Biota in western Liaoning and adjacent areas in the last two decades, such as Sinovenator, Mei, Sinusonasus, Jinfengopteryx, Daliansaurus, Liaoningvenator, and Jianianhualong (Xu et al, 2002, 2017; Xu & Norell, 2004; Xu & Wang, 2004; Ji et al, 2005; Shen et al, 2017a, 2017b)
Identification of PMOL-AD00102 as Sinovenator changii and comparisons with other Jehol troodontids PMOL-AD00102 can be assigned to the Troodontidae based on the combination of individual characters that are typical of troodontids and/or have been regarded as synapomorphies for troodontids in different studies (Makovicky & Norell, 2004; Xu et al, 2017): a row of foramina along a longtitudinal line on the nasal; a well-developed supraorbital crest that expands laterally anterodorsal to the orbit on the lacrimal; a lateral ridge close to the ventral edge of the jugal; a pit on the ventral surface of the laterosphenoid; a reduced basal tubera that lie directly ventral to the occipital condyle; an oval-shaped foramen magnum; the quadrate bears a pneumatic fenestra and a lateral groove on the dentary
Summary
Troodontidae is a group of small to middle-bodied theropod dinosaurs, and is well known from the Cretaceous rocks of Asia and North America (Makovicky & Norell, 2004) It has a high morphological relevance in understanding the avian origin (Xu et al, 2002). Many exquisitely preserved troodontid fossils have been reported from the Early Cretaceous Jehol Biota in western Liaoning and adjacent areas in the last two decades, such as Sinovenator, Mei, Sinusonasus, Jinfengopteryx, Daliansaurus, Liaoningvenator, and Jianianhualong (Xu et al, 2002, 2017; Xu & Norell, 2004; Xu & Wang, 2004; Ji et al, 2005; Shen et al, 2017a, 2017b) These discoveries shed new lights on the evolution of troodontids and the origin of birds (Xu et al, 2002, 2017; Xu & Norell, 2004).
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